Photon-counting X-ray CT apparatuses detect the photon count and the energy level of the X-rays incident on each of the detection elements, generates a CT image for each of the different energy levels, and decomposes substances and estimates the substance densities. This is also referred to as multi-energy reconstruction.
When a large number of photons become incident on the detection elements, however, such a photon-counting X-ray CT apparatus sometimes falls incapable of decomposing the substances or estimating the substance densities appropriately. This is because the waveforms generated by the individual photons overlap one another, due to the response time of the detection elements, and, as a result, a plurality of photons are counted as one photon. This phenomenon is referred to as pulse pile-up.